Antifuse OTP structures with hybrid low-voltage devices

ABSTRACT

An antifuse One-Time-Programmable memory cell includes a substrate, and a hybrid select transistor and a hybrid antifuse capacitor formed on the substrate. The hybrid select transistor includes a first gate dielectric layer formed on the substrate, wherein the first gate dielectric layer is thinner than 40 nm, a first high-voltage junction formed in the substrate, and a low-voltage junction formed in the substrate. The hybrid antifuse capacitor includes a second gate dielectric layer, wherein the second gate dielectric layer is thinner than 40 nm, which enables a low-voltage antifuse capacitor device, a second gate formed on the gate dielectric layer, a second high-voltage junction formed in the substrate, and a third high-voltage junction formed in the substrate.

BACKGROUND OF THE INVENTION

The present invention relates to the technical field of semiconductormemory, and in particular, to One Time Programmable (OTP) memory.

Embedded Non-Volatile Memory (ENVM) technology has been employed in anumber of applications such as post silicon tuning, memory repair,on-line field test, and secure ID storage. ENVM is also a criticalcomponent for self-healing applications in which information regardingtime dependent failure mechanisms such as circuit aging must be retainedduring system power off periods. Anti-Fuse One Time Programmable(AF-OTP) NVM memory has been extensively used for memory repair instandard logic processes.

An antifuse is non-conductive in the native unprogrammed state andbecomes conductive when programmed, hence the name antifuse. Inintegrated circuit, antifuse is commonly constructed with a thindielectric layer sandwiched between two conductors. To program anantifuse, a high voltage is applied between the two conductors. Thisleads to a physical and permanent breakdown of the thin dielectric layerand the formation of a current conduction path between the twoconductors. Antifuse can thus be used as a memory element. Programmedstate of an antifuse represents data “1” and unprogrammed state “0” orvice versa. Once programmed, antifuse memory cannot revert tounprogrammed state, i.e., it is a one-time programmable memory. Antifuseretains conductive or non-conductive state even after the power isturned off, rendering the data non-volatile. As such, antifuse memory isa non-volatile OTP memory.

In order to provide a high voltage to program the antifuse capacitor,the select transistor is usually built with thick gate dielectric layerin the device, and with high voltage junctions. However, high-voltagetransistor devices with high voltage junctions will take more siliconarea because wider channel length and width are needed, and the moresilicon area will directly correlate to higher cost and lowerintegration density. On the other hand, the antifuse capacitor isusually built with a thinner gate dielectric layer for easierprogramming, and low-voltage junctions are usually applied inconjunction with its low-voltage device.

There is therefore a need for antifuse OTP memory cells with highstructural simplicity, and high integration density.

SUMMARY OF THE INVENTION

In view of the above, the present disclosure provides an antifuse OTPmemory bit cell that includes an antifuse capacitor and ametal-oxide-semiconductor field-effect-transistor (MOSFET). The MOSFETis a select transistor which is implemented to program or access theantifuse capacitor. A novel memory cell includes a hybrid structure: alow voltage gate dielectric layer, and a high voltage junction or amixed high-voltage and low-voltage junctions, for smaller silicon footprint. A novel select transistor includes a low-voltage gate dielectriclayer, and a low-voltage junction or a mixed high-voltage andlow-voltage junctions, or high-voltage junctions, for both high voltageprogramming and smaller silicon area.

In one general aspect, the present invention relates to an antifuseOne-Time-Programmable memory cell, which includes a substrate; a hybridselect transistor formed on the substrate that includes: a first gatedielectric layer formed on the substrate, wherein the first gatedielectric layer is thinner than 40 nm, which enables a low-voltageselect transistor device; a first gate formed on the gate dielectriclayer; a first high-voltage junction formed in the substrate; and alow-voltage junction formed in the substrate, wherein a source and adrain for the select transistor are formed by the first high-voltagejunction and the low-voltage junction; and a hybrid antifuse capacitorformed on the substrate that includes a second gate dielectric layerformed on the substrate, wherein the second gate dielectric layer isthinner than 40 nm, which enables a low-voltage antifuse capacitordevice; a second gate formed on the gate dielectric layer; a secondhigh-voltage junction formed in the substrate; and a third high-voltagejunction formed in the substrate, wherein a source and a drain for theantifuse capacitor are respectively formed by the second high-voltagejunction and the third high-voltage junction.

Implementations of the system may include one or more of the following.The second high-voltage junction and the third high-voltage junction ofthe antifuse capacitor can be separated by a channel in the substrate.The second high-voltage junction and the third high-voltage junction ofthe antifuse capacitor can be electrically connected. The first gatedielectric layer or the second gate dielectric layer can be thinner than30 nm. The hybrid select transistor can be a low voltage deviceoperating at 3.3V or below. The hybrid antifuse capacitor can be a lowvoltage device operating at 3.3V or below. The first high-voltagejunction or the second high-voltage junction or the third high-voltagejunction can have a threshold junction breakdown voltage above 3.3 Volt.The first high-voltage junction or the second high-voltage junction orthe third high-voltage junction can have a threshold junction breakdownvoltage above 5 Volt. The second high-voltage junction and the thirdhigh-voltage junction can be electrically connected. The low-voltagejunction can include a first doped region adjacent to the first gatedielectric layer at a lower doped level than a second doped region awayfrom the first gate dielectric layer. The first high-voltage junctioncan include a first doped region adjacent to the first gate dielectriclayer at a lower doped level than a second doped region away from thefirst gate dielectric layer. The first high-voltage junction can alsohave a substantially uniform doping level. The second high-voltagejunction or the third high-voltage junction can include a first dopedregion adjacent to the second gate dielectric layer at a lower dopedlevel than a second doped region away from the second gate dielectriclayer. The second high-voltage junction or the third high-voltagejunction can have a substantially uniform doping level.

In another general aspect, the present invention relates to an antifuseOne-Time-Programmable memory cell, which includes: a substrate; alow-voltage select transistor formed on the substrate which includes: afirst gate dielectric layer formed on the substrate, wherein the firstgate dielectric layer is thinner than 40 nm, which enables a low-voltageselect transistor device; a first gate formed on the gate dielectriclayer; a first low-voltage junction formed in the substrate; and asecond low-voltage junction formed in the substrate, wherein a sourceand a drain for the select transistor are formed by the firstlow-voltage junction and the second low-voltage junction; and a hybridantifuse capacitor formed on the substrate, which includes: a secondgate dielectric layer formed on the substrate, wherein the second gatedielectric layer is thinner than 40 nm, which enables a low-voltageantifuse capacitor device; a second gate formed on the gate dielectriclayer; a first high-voltage junction formed in the substrate; and asecond high-voltage junction formed in the substrate, wherein a sourceand a drain for the antifuse capacitor are respectively formed by thefirst high-voltage junction and the second high-voltage junction.

Implementations of the system may include one or more of the following.The first high-voltage junction and the second high-voltage junction ofthe antifuse capacitor can be separated by a channel in the substrate.The first high-voltage junction and the second high-voltage junction ofthe antifuse capacitor are electrically connected. The first gatedielectric layer or the second gate dielectric layer can be thinner than30 nm. The hybrid select transistor can be a low voltage deviceoperating at 3.3V or below. The hybrid antifuse capacitor can be a lowvoltage device operating at 3.3V or below. The first high-voltagejunction or the second high-voltage junction can have a thresholdjunction breakdown voltage above 3.3 Volt. The second low-voltagejunction and the first high-voltage junction can be electricallyconnected. The first low-voltage junction or the second low-voltagejunction can include a first doped region adjacent to the first gatedielectric layer at a lower doped level than a second doped region awayfrom the first gate dielectric layer. The first high-voltage junction orthe second high-voltage junction can include a first doped regionadjacent to the second gate dielectric layer at a lower doped level thana second doped region away from the second gate dielectric layer.

BRIEF DESCRIPTION OF THE DRAWINGS

To illustrate the technical solutions of the present invention, theaccompanying drawings briefly described embodiments need to be used indescribing the embodiments. Obviously, the following drawings onlydescribe examples of the present invention. For those skilled in theart, other drawings may also be obtained according to these drawingswithout any creative work.

FIG. 1A is a cross-sectional view of an antifuse OTP memory cell havinga hybrid low-voltage antifuse capacitor and a hybrid low-voltage selecttransistor in accordance with some embodiments of the present invention.

FIG. 1B is a schematic planar drawing of the antifuse OTP memory cellshown in FIG. 1A.

FIG. 2 is a cross-sectional view of the antifuse OTP memory cell relatedto the antifuse OTP memory cell in FIG. 1A.

FIG. 3 is a cross-sectional view of another antifuse OTP memory cellhaving a hybrid low-voltage antifuse capacitor and a low-voltage selecttransistor in accordance with some embodiments of the present invention.

FIG. 4 is a cross-sectional view of the antifuse OTP memory cell relatedto the antifuse OTP memory cell in FIG. 3.

FIG. 5 is a cross-sectional view of another antifuse OTP memory cellhaving a hybrid low-voltage antifuse capacitor and a hybrid low-voltageselect transistor in accordance with some embodiments of the presentinvention.

FIG. 6 is a cross-sectional view of the antifuse OTP memory cell relatedto the antifuse OTP memory cell in FIG. 5.

FIG. 7 is a cross-sectional view of another antifuse OTP memory cellhaving a hybrid low-voltage antifuse capacitor and a hybrid low-voltageselect transistor in accordance with some embodiments of the presentinvention.

FIG. 8 is a process flow chart to build the antifuse OTP memory cellshown FIGS. 1-7.

FIG. 9 is a cross-sectional view of another antifuse OTP memory cellhaving a hybrid low-voltage antifuse capacitor and a hybrid low-voltageselect transistor related to FIGS. 1A and 1B.

FIG. 10 is a cross-sectional view of another antifuse OTP memory cellhaving a hybrid low-voltage antifuse capacitor and a hybrid low-voltageselect transistor related to FIG. 2.

FIG. 11 is a process flow chart to build the antifuse OTP memory cellshown FIGS. 9-10.

DETAILED DESCRIPTION OF THE INVENTION

The technical solutions in the embodiments of the present invention aredescribed with reference to the accompanying drawings in the embodimentsof the present invention.

In some embodiments, the antifuse OTP memory usesmetal-oxide-semiconductor (MOS) capacitor as the antifuse element. TheMOS capacitor is connected to an access device called select transistor,typically a metal-oxide-semiconductor field-effect-transistor (MOSFET)to form an antifuse memory bit cell. Referring to FIGS. 1A-1B, anantifuse OTP memory cell 100 includes a select transistor 110 and anantifuse capacitor 120. The select transistor 110 includes a gate 115 ona gate dielectric 116 on a substrate 105. The gate dielectric 116includes a thin dielectric layer, which makes the select transistor 110a low-voltage device. For silicon oxide, a thin dielectric layer istypically below 40 angstroms, and can further be thinner than 30angstroms. The antifuse capacitor 120 includes a gate 125 on a gatedielectric 126 on the substrate 105. The gate dielectric 126 includes athin dielectric layer. A thin dielectric layer is typically below 40angstroms for silicon oxide, and can further be thinner than 30angstroms, which makes the antifuse capacitor 120 also a low-voltagedevice. A low voltage device can operate at 3.3V or below, or further at2.5V or below.

The junctions (e.g. the source or the drain) of the select transistor110 are formed respectively by lightly doped regions 111, 112 and thedeeply doped regions 113, 114. The two junctions are separated in thesubstrate 105.

The junctions (e.g. source or drain) of the antifuse capacitor 120 areformed respectively by lightly doped regions 121, 122 and the deeplydoped regions 123, 124. The junctions (respectively formed by the dopedregions 122, 123 and the doped regions 121, 124) of the antifusecapacitor 120 are separated by a channel 130 and not connected in thesubstrate 105 (in contrast to the example described in FIG. 2 below).

The combination of lightly doped regions 111, 112 and 121, 122 anddeeply doped regions 113, 114 and 123, 124 is formed on both sides ofthe channels by applying the spacer structures 117, 127 between twodoping processes.

The source/drain and the gates 115, 125 of the select transistor 110 andthe antifuse capacitor 120 are of the same conductivity type. Thetransistor channel region, i.e., the silicon substrate 105 underneathgate dielectric is of the opposite conductivity type. In the selecttransistor 110, for example, if the gate 115 and source/drainrespectively formed by doped regions 111, 114 and doped regions 112, 113are N type, the regions of the substrate 105 underneath the gatedielectric 116 is P type. Thus, the source/drain regions form PNjunctions with the oppositely doped silicon substrate 105. Similarly,for the antifuse capacitor 120, if the gate 125 and source/drainrespectively formed by doped regions 121, 124 and 122, 123 are N type,the region of the substrate 105 underneath the gate dielectric 126 is Ptype.

The doped regions 111, 112 and 121, 122 are lightly doped, wherein theelectrically active concentration of dopant level can be in a range of1×10¹⁷˜1×10¹⁹ atoms/cm³. The doped regions 113, 114 and 123, 124 formedon both sides of the channels are deeply doped, wherein the electricallyactive concentration of dopant level can be above 2×10¹⁹ atoms/cm⁻³.

Doped regions of 113, 112 are connected, and they are electricallyconductive. Similarly, doped regions 111, 114, 124, and 121 areconnected, and they are electrically conductive. Doped regions 122, 123are also connected. During antifuse programming, a voltage is applied onthe gate 115 to turn on the select transistor 110, i.e., to form aconductive channel in the substrate 105 underneath the gate dielectric116. In the meantime, a programming voltage is applied to the dopedregion 113. Through the conductive path underneath the gate dielectric116, the programming voltage is actually applied on the gate dielectric126 to cause a dielectric breakdown. A low electrical resistance will bemeasured between the gate 125 and the doping region 124 or the dopingregion 123.

In order for transistor to function normally, the junction bias needs tobe below junction breakdown voltage. For low voltage transistor, ashallower and steeper junction can be used without causing breakdown.Thus the device can have better short-channel behaviors. Formation ofthis kind of junction requires shallower implants, lower temperature andshorter anneals. For high-voltage transistor, it is critical to avoid apeak junction electric field that may cause breakdown; therefore thejunction profile must be adjusted accordingly. To avoid introducing avery high peak field at the junction, more gradual junction dopingprofiles are preferred. This usually means deeper implants, longer(maybe multiple) annealing steps. This kind of junctions consequentlycan handle higher bias voltages.

The gate 115 and the gate dielectric 116 in the select transistor 110support a low-voltage device for high programming yield. The dopedregions 111/114 form a high-voltage (HV) junction, while doped regions112/113 form a low-voltage (LV) junction with the substrate 105 for theselect transistor 110. The breakdown threshold voltage for HV junctionscan be higher than 3.3V, or higher than 5V depending on technology nodeof the device. The breakdown threshold voltage for LV junctions can belower than 3.2V, or below 2.3V depending on technology node of thedevice.

An important aspect of the antifuse OTP memory cell 100 is that both thegate 125 and the gate dielectric 126 in the antifuse capacitor 120 andthe gate 115 and the gate dielectric 116 in the select transistor 110support low-voltage devices, which reduce silicon foot print. Moreover,the select transistor 110 includes a low-voltage (LV) junction formed bydoped regions 112/113, which also reduces silicon foot print. The otherhigh-voltage junction formed by doped regions 111/114 enables the selecttransistor 110 to have an improved programming yield.

In the antifuse capacitor 120, the doped regions 122/123 and 121/124form HV junctions with the substrate 105. The junction breakdownthreshold voltage for HV junctions can be higher than 3.3V, or higherthan 5V depending on technology node of the device. The combination oflow voltage gate dielectric layer (i.e. an LV device) and a high voltagejunction provides low silicon foot print and high programming yield.

Since HV junctions are formed for both the antifuse capacitor 120 andthe select transistor 110, the deeply doped regions 114 and 124 can becombined to simplify the structure and process. Thus, the HV junctionformed by doped regions 111, 114 for the select transistor 110 and theHV junction formed by doped regions 121, 124 for the antifuse capacitor120 are conductively connected.

In some embodiments, referring to FIG. 2, an antifuse OTP memory cell200 includes a select transistor 110 and an antifuse capacitor 120 andassociated components, similar to the antifuse OTP memory cell 100 asdescribed above. A difference from the antifuse OTP memory cell 100 isthat the lightly doped regions 121, 122 in the antifuse OTP memory cell200 are electrically connected instead of separated by a channel.

An antifuse is non-conductive in the native unprogrammed state andbecomes conductive when programmed. Usually, a voltage is applied on theantifuse capacitor, e.g., between the deeply doped regions 123 and gate125. An electric current is measured to determine if the antifuse memorycell is programmed or not. However, if the lightly doped regions 121 and122 are not connected in the substrate 105 underneath the gate oxide 126(i.e. separated by the channel 130 in FIG. 1A), part of the electriccurrent will leak into the substrate 105 through the channel 130 shownin FIG. 1A and the current may not be stable. By merging the dopant area121 and 122, the current leakage path into the substrate 105 is blocked,and current measurement noise can be effectively reduced.

In some embodiments, referring to FIG. 3, an antifuse OTP memory cell300 includes a select transistor 310 and an antifuse capacitor 320. Theselect transistor 310 includes a gate 315 on a gate dielectric 316 on asubstrate 305. The gate dielectric 316 includes a thin dielectric layer,which makes the select transistor 310 a low-voltage device. For siliconoxide, a thin dielectric layer can be 40 angstroms or thinner, or canfurther be further thinner than 30 angstroms. The antifuse capacitor 320includes a gate 325 on a gate dielectric 326 on the substrate 305. Thegate dielectric 326 includes a thin dielectric layer, which makes theantifuse capacitor 320 also a low-voltage device.

The junctions (e.g. the source or the drain) of the select transistor310 are formed respectively by lightly doped regions 311, 312 and thedeeply doped regions 313, 314. The two junctions are separated in thesubstrate 305.

The junctions (e.g. source or drain) of the antifuse capacitor 320 areformed respectively by lightly doped regions 321, 322 and the deeplydoped regions 323, 324. The junctions (respectively formed by the dopedregions 322, 323 and the doped regions 321, 324) of the antifusecapacitor 320 are separated by a channel 330 and not connected in thesubstrate 305 (in contrast to the example described in FIG. 4 below).

The combination of lightly doped regions 311, 312 and 321, 322 anddeeply doped regions 313, 314 and 323, 324 is formed on both sides ofthe channels by applying the spacer structures 317, 327 between twodoping processes.

The source/drain and the gates 315, 325 of the select transistor 310 andthe antifuse capacitor 320 are of the same conductivity type. Thetransistor channel region, i.e., the silicon substrate 305 underneathgate dielectric is of the opposite conductivity type. In the selecttransistor 310, for example, if the gate 315 and source/drainrespectively formed by doped regions 311, 314 and doped regions 312, 313are N type, the regions of the substrate 305 underneath the gatedielectric 316 is P type. Thus, the source/drain regions form PNjunctions with the oppositely doped silicon substrate 305. Similarly,for the antifuse capacitor 320, if the gate 325 and source/drainrespectively formed by doped regions 321, 324 and 322, 323 are N type,the region of the substrate 305 underneath the gate dielectric 326 is Ptype.

The doped regions 311, 312 and 321, 322 are lightly doped, wherein theelectrically active concentration of dopant level can be in a range of1×10¹⁷˜1×10¹⁹ atoms/cm³. The doped regions 313, 314 and 323, 324 formedon both sides of the channels are deeply doped, wherein the electricallyactive concentration of dopant level can be above 2×10¹⁹ atoms/cm⁻³.

Doped regions of 313, 312 are connected, and they are electricallyconductive. Similarly, doped regions 311, 314, 324, and 321 areconnected, and they are electrically conductive. Doped regions 322, 323are also connected. During antifuse programming, a voltage is applied onthe gate 315 to turn on the select transistor 310, i.e., to form aconductive channel in the substrate 305 underneath the gate dielectric316. In the meantime, a programming voltage is applied to the dopedregion 313. Through the conductive path underneath the gate dielectric316, the programming voltage is actually applied on the gate dielectric326 to cause a dielectric breakdown. A low electrical resistance will bemeasured between the gate 325 and the doping region 324 or between thegate 325 and the doping region 323.

The gate 315 and the gate dielectric 316 in the select transistor 310support a low-voltage device for area scaling. The doped regions 311/314and doped regions 312/313 form a low-voltage (LV) junction with thesubstrate 305 for the select transistor 310. The breakdown thresholdvoltage for HV junctions can be higher than 3.3V, or higher than 5Vdepending on technology node of the device. The breakdown thresholdvoltage for LV junctions can be lower than 3.2V, or below 2.3V dependingon technology node of the device.

An important aspect of the antifuse OTP memory cell 300 is that both thegate 325 and the gate dielectric 326 in the antifuse capacitor 320 andthe gate 315 and the gate dielectric 316 in the select transistor 310support low-voltage devices, which reduce silicon foot print. Moreover,the select transistor 310 includes low-voltage (LV) junctions formed bydoped regions 312/313 and 311/314, which also reduces silicon footprint.

In some embodiments, referring to FIG. 4, an antifuse OTP memory cell400 includes a select transistor 310 and an antifuse capacitor 320 andassociated components, similar to the antifuse OTP memory cell 300 asdescribed above in relation to FIG. 3. A difference from the antifuseOTP memory cell 300 is that the lightly doped regions 321, 322 in theantifuse OTP memory cell 400 are electrically connected instead ofseparated by a channel.

An antifuse is non-conductive in the native unprogrammed state andbecomes conductive when programmed. Usually, a voltage is applied on theantifuse capacitor, e.g., between the deeply doped region 323 and gate325 or between the deeply doped region 324 and gate 325. An electriccurrent is measured to determine if the antifuse memory cell isprogrammed or not. However, if the lightly doped regions 321 and 322 arenot connected in the substrate 305 underneath the gate oxide 326 (i.e.separated by the channel 330 shown in FIG. 3), part of the electriccurrent will leak into the substrate 305 through the channel 330 (FIG.3); the current measurement will not be stable. By merging the dopantarea 321 and 322, the current leakage path into the substrate 305 isblocked, and current measurement noise can be effectively reduced.

In some embodiments, referring to FIG. 5, an antifuse OTP memory cell500 includes a select transistor 510 and an antifuse capacitor 520. Theselect transistor 510 includes a gate 515 on a gate dielectric 516 on asubstrate 505. The gate dielectric 516 includes a thin dielectric layer,which makes the select transistor 510 a low-voltage device. For siliconoxide, a thin dielectric layer can be 40 angstroms or thinner, or canfurther be further thinner than 30 angstroms. The antifuse capacitor 520includes a gate 525 on a gate dielectric 526 on the substrate 505. Thegate dielectric 526 includes a thin dielectric layer, which makes theantifuse capacitor 520 also a low-voltage device.

The junctions (e.g. the source or the drain) of the select transistor510 are formed respectively by lightly doped regions 511, 512 and thedeeply doped regions 513, 514. The two junctions are separated in thesubstrate 505.

The junctions (e.g. source or drain) of the antifuse capacitor 520 areformed respectively by lightly doped regions 521, 522 and the deeplydoped regions 523, 524. The junctions (respectively formed by the dopedregions 522, 523 and the doped regions 521, 524) of the antifusecapacitor 520 are separated by a channel 530 and not connected in thesubstrate 505 (in contrast to the example described in FIG. 6 below).

The combination of lightly doped regions 511, 512 and 521, 522 anddeeply doped regions 513, 514 and 523, 524 is formed on both sides ofthe channels by applying the spacer structures 517, 527 between twodoping processes.

The source/drain and the gates 515, 525 of the select transistor 510 andthe antifuse capacitor 520 are of the same conductivity type. Thetransistor channel region, i.e., the silicon substrate 505 underneathgate dielectric is of the opposite conductivity type. In the selecttransistor 510, for example, if the gate 515 and source/drainrespectively formed by doped regions 511, 514 and doped regions 512, 513are N type, the regions of the substrate 505 underneath the gatedielectric 516 is P type. Thus, the source/drain regions form PNjunctions with the oppositely doped silicon substrate 505. Similarly,for the antifuse capacitor 520, if the gate 525 and source/drainrespectively formed by doped regions 521, 524 and 522, 523 are N type,the region of the substrate 505 underneath the gate dielectric 526 is Ptype.

The doped regions 511, 512 and 521, 522 are lightly doped, wherein theelectrically active concentration of dopant level can be in a range of1×10¹⁷˜1×10¹⁹ atoms/cm³. The doped regions 513, 514 and 523, 524 formedon both sides of the channels are deeply doped, wherein the electricallyactive concentration of dopant level can be above 2×10¹⁹ atoms/cm⁻³.

Doped regions of 513, 512 are connected, and they are electricallyconductive. Similarly, doped regions 511, 514, 524, and 521 areconnected, and they are electrically conductive. Doped regions 522, 523are also connected. During antifuse programming, a voltage is applied onthe gate 515 to turn on the select transistor 510, i.e., to form aconductive channel in the substrate 505 underneath the gate dielectric516. In the meantime, a programming voltage is applied to the dopedregion 513. Through the conductive path underneath the gate dielectric516, the programming voltage is actually applied on the gate dielectric526 to cause a dielectric breakdown. A low electrical resistance will bemeasured between the gate 525 and the doping region 524 or between thegate 525 and the doping region 523.

The gate 515 and the gate dielectric 516 in the select transistor 510support a low-voltage device for high programming yield. The dopedregions 511/514 and the doped regions 512/513 form high-voltage (HV)junctions with the substrate 505 for the select transistor 510. Thebreakdown threshold voltage for HV junctions can be higher than 3.3V, orhigher than 5V depending on technology node of the device. The breakdownthreshold voltage for LV junctions can be lower than 3.2V, or below 2.3Vdepending on technology node of the device.

An important aspect of the antifuse OTP memory cell 500 is that both thegate 525 and the gate dielectric 526 in the antifuse capacitor 520 andthe gate 515 and the gate dielectric 516 in the select transistor 510support low-voltage devices, which reduce silicon foot print. Moreover,the high-voltage junctions in the select transistor 510 can provideincreased programming yield.

In some embodiments, referring to FIG. 6, an antifuse OTP memory cell600 includes a select transistor 510 and an antifuse capacitor 520 andassociated components, similar to the antifuse OTP memory cell 500 asdescribed above in relation to FIG. 5. A difference from the antifuseOTP memory cell 500 is that the lightly doped regions 521, 522 in theantifuse OTP memory cell 600 are electrically connected instead ofseparated by a channel.

An antifuse is non-conductive in the native unprogrammed state andbecomes conductive when programmed. Usually, a voltage is applied on theantifuse capacitor, e.g., between the deeply doped region 523 and gate525 or between the deeply doped region 524 and gate 525. An electriccurrent is measured to determine if the antifuse memory cell isprogrammed or not. However, if the lightly doped regions 521 and 522 arenot connected in the substrate 505 underneath the gate oxide 526 (i.e.separated by the channel 530 shown in FIG. 5), part of the electriccurrent will leak into the substrate 505 through the channel 530 (FIG.5); the current measurement will not be stable. By merging the dopantarea 521 and 522, the current leakage path into the substrate 505 isblocked, and current measurement noise can be effectively reduced.

In some embodiments, referring to FIG. 7, an antifuse OTP memory cell700 includes a select transistor 710 and an antifuse capacitor 720. Theselect transistor 710 includes a gate 715 on a gate dielectric 716 on asubstrate 705. The gate dielectric 716 includes a thin dielectric layer,which makes the select transistor 710 a low-voltage device. For siliconoxide, a thin dielectric layer can be 40 angstroms or thinner, or canfurther be further thinner than 30 angstroms. The antifuse capacitor 720includes a gate 725 on a gate dielectric 726 on the substrate 705. Thegate dielectric 726 includes a thin dielectric layer, which makes theantifuse capacitor 720 also a low-voltage device.

The junctions (e.g. the source or the drain) of the select transistor710 are formed respectively by lightly doped regions 711, 712 and thedeeply doped regions 713, 714. The two junctions are separated in thesubstrate 705.

The junctions (e.g. source or drain) of the antifuse capacitor 720 areformed respectively by lightly doped regions 721, 722 and the deeplydoped regions 723, 724. The junctions (respectively formed by the dopedregions 722, 723 and the doped regions 721, 724) of the antifusecapacitor 720 are separated by a channel 730 and not connected in thesubstrate 705.

The combination of lightly doped regions 711, 712 and 721, 722 anddeeply doped regions 713, 714 and 723, 724 is formed on both sides ofthe channels by applying the spacer structures 717, 727 between twodoping processes.

The source/drain and the gates 715, 725 of the select transistor 710 andthe antifuse capacitor 720 are of the same conductivity type. Thetransistor channel region, i.e., the silicon substrate 705 underneathgate dielectric is of the opposite conductivity type. In the selecttransistor 710, for example, if the gate 715 and source/drainrespectively formed by doped regions 711, 714 and doped regions 712, 713are N type, the regions of the substrate 705 underneath the gatedielectric 716 is P type. Thus, the source/drain regions form PNjunctions with the oppositely doped silicon substrate 705. Similarly,for the antifuse capacitor 720, if the gate 725 and source/drainrespectively formed by doped regions 721, 724 and 722, 723 are N type,the region of the substrate 705 underneath the gate dielectric 726 is Ptype.

The doped regions 711, 712 and 721, 722 are lightly doped, wherein theelectrically active concentration of dopant level can be in a range of1×10¹⁷˜1×10¹⁹ atoms/cm³. The doped regions 713, 714 and 723, 724 formedon both sides of the channels are deeply doped, wherein the electricallyactive concentration of dopant level can be above 2×10¹⁹ atoms/cm⁻³.

Doped regions of 713, 712 are connected, and they are electricallyconductive. Similarly, doped regions 711, 714, 724, and 721 areconnected, and they are electrically conductive. Doped regions 722, 723are also connected. During antifuse programming, a voltage is applied onthe gate 715 to turn on the select transistor 710, i.e., to form aconductive channel in the substrate 705 underneath the gate dielectric716. In the meantime, a programming voltage is applied to the dopedregion 713. Through the conductive path underneath the gate dielectric716, the programming voltage is actually applied on the gate dielectric726 to cause a dielectric breakdown. A low electrical resistance will bemeasured between the gate 725 and the doping region 724 or the dopingregion 723.

In order for transistor to function normally, the junction bias needs tobe below junction breakdown voltage. For low voltage transistor, ashallower and steeper junction can be used without causing breakdown.Thus the device can have better short-channel behaviors. Formation ofthis kind of junction requires shallower implants, lower temperature andshorter anneals. For high-voltage transistor, it is critical to avoid apeak junction electric field that may cause breakdown; therefore thejunction profile must be adjusted accordingly. To avoid introducing avery high peak field at the junction, more gradual junction dopingprofiles are preferred. This usually means deeper implants, longer(maybe multiple) annealing steps. This kind of junctions consequentlycan handle higher bias voltages.

In the select transistor 710, the gate 715 and the gate dielectric 716support a low-voltage device for high programming yield. The dopedregions 711/714 form a high-voltage (HV) junction, while doped regions712/713 form a low-voltage (LV) junction with the substrate 705 for theselect transistor 710. The breakdown threshold voltage for HV junctionscan be higher than 3.3V, or higher than 5V depending on technology nodeof the device. The breakdown threshold voltage for LV junctions can belower than 3.2V, or below 2.3V depending on technology node of thedevice.

An important aspect of the antifuse OTP memory cell 700 is that both thegate 725 and the gate dielectric 726 in the antifuse capacitor 720 andthe gate 715 and the gate dielectric 716 in the select transistor 710support low-voltage devices, which reduce silicon foot print. Moreover,the select transistor 710 includes a low-voltage (LV) junction formed bydoped regions 712/713, which also reduces silicon foot print. The otherhigh-voltage junction formed by doped regions 711/714 enables the selecttransistor 710 to have an improved programming yield.

In the antifuse capacitor 720, the doped regions 721/724 form HVjunctions with the substrate 705 while the doped regions 722/723 form LVjunctions with the substrate 705. The breakdown threshold voltage for HVjunctions can be higher than 3.3V, or higher than 5V depending ontechnology node of the device. The breakdown threshold voltage for LVjunctions can be lower than 3.2V, or below 2.3V depending on technologynode of the device. The low voltage gate dielectric layer (i.e. an LVdevice), a low voltage junction provides low silicon foot print, whilethe high voltage junction enables high programming yield.

Since HV junctions are formed for both the antifuse capacitor 720 andthe select transistor 710, the deeply doped regions 714 and 724 can becombined to simplify the structure and process. Thus, the HV junctionformed by doped regions 711, 714 for the select transistor 710 and theHV junction formed by doped regions 721, 724 for the antifuse capacitor720 are conductively connected.

Referring to FIG. 8, a process to build the above described antifuse OTPmemory cells can include the following steps. Using the antifuse OTPmemory cell 100 as an example, first, a gate dielectric layer is formedon a substrate (step 180). Gate dielectric and gate patterns are thenproduced for an antifuse capacitor and a select transistor on thesubstrate (step 182). These can be accomplished using masking andetching techniques. A thin gate dielectric layer is formed on thesubstrate for the antifuse capacitor and a thicker gate dielectric layeris formed on the substrate for the select transistor respectively. Then,the gate stacks are formed on top of the gate dielectric layers. Theantifuse capacitor and the select transistor are defined by masking andetching. The combination of lightly doped regions and deeply dopedregions is formed by applying the spacer structures 117, 127 between twodoping processes.

The relatively shallow doping regions (e.g. 121, 122 in FIGS. 1A and 1B)for the antifuse capacitor 120 and the relatively shallow doping regions(e.g. 112, 112 in FIGS. 1A and 1B) for the select transistor 110 areformed through a single masking and ion implants or separate masking andion implants (step 184). Moreover, annealing steps maybe added afterdopant implant process.

Next, spacers are built around the antifuse capacitor stack and theselect transistor gate stack (step 186). Deeply doped regions 113, 114(FIGS. 1A and 1B) for the select transistor 110, and deeply dopedregions 123, 124 (FIGS. 1A and 1B) for the antifuse capacitor 120 areformed in masking process and ion implant (step 188).

The above described only serves as examples of critical process steps tobuild an antifuse OTP memory cell shown in FIGS. 1A-1B. Additionalprocess steps maybe included between processes steps described above,and other process steps are needed before and after the processes stepsdescribed above for building an entire device. For example, in steps182-188, the source and drain junctions can be formed by ionimplantation.

In some embodiments, the high-voltage junctions of an antifuse OTPmemory cell can be formed by deeply doped regions in the substratewithout using the lightly doped regions. Referring to FIG. 9, an OTPmemory cell 900 includes a hybrid low-voltage antifuse capacitor and ahybrid low-voltage select transistor, similar to the OTP memory cell 100in FIGS. 1A and 1B, except that the light doped regions (111, 121, 122in FIGS. 1A and 1B) are not formed, and anti-punch through ion implantis eliminated in these regions. The light doped region 112 is kept.

The gate dielectric 116 includes a thin dielectric layer, which makesthe select transistor 110 a low-voltage device. The antifuse capacitor120 includes a gate 125 on a gate dielectric 126 on the substrate 105.The gate dielectric 126 includes a thin dielectric layer, which makesthe antifuse capacitor 120 also a low-voltage device. For silicon oxide,a thin dielectric layer is typically below 40 angstroms, and can furtherbe thinner than 30 angstroms. A low voltage device can operate at 3.3Vor below, or further at 2.5V or below.

The junctions (e.g. the source or the drain) of the select transistor110 are formed by doped regions 112/113 (low-voltage junction) and thedoped region 114 (high-voltage junction) respectively. The two junctionsare separated in the substrate 105. The high-voltage junctions (e.g.source or drain) of the antifuse capacitor 120 are formed respectivelyby deeply doped regions 123, 124. The high-voltage junctions(respectively formed by the deeply doped region 123 and the deeply dopedregion 124) of the antifuse capacitor 120 are separated by a channel 130and not connected in the substrate 105 (in contrast to the exampledescribed in FIG. 10 below). The deeply doped regions 114, 123, 124 eachhave a substantially uniform doping level.

In some embodiments, referring to FIG. 10, an OTP memory cell 1000includes a hybrid low-voltage antifuse capacitor and a hybridlow-voltage select transistor, similar to the OTP memory cell 200 inFIG. 2, except that the light doped regions (111, 121, 122 in FIG. 2)are not formed. The light doped region 112 is kept.

The gate dielectric 116 includes a thin dielectric layer, which makesthe select transistor 110 a low-voltage device. The antifuse capacitor120 includes a gate 125 on a gate dielectric 126 on the substrate 105.The gate dielectric 126 includes a thin dielectric layer, which makesthe antifuse capacitor 120 also a low-voltage device. For silicon oxide,a thin dielectric layer is typically below 40 angstroms, and can furtherbe thinner than 30 angstroms. A low voltage device can operate at 3.3Vor below, or further at 2.5V or below.

The junctions (e.g. the source or the drain) of the select transistor110 are formed by doped regions 112/113 (low-voltage junction) and thedoped region 114 (high-voltage junction) respectively. The two junctionsare separated in the substrate 105. The junctions (e.g. source or drain)of the antifuse capacitor 120 are formed respectively by deeply dopedregions 123, 124. The high-voltage junctions (respectively formed by thedeeply doped region 123 and the deeply doped region 124) of the antifusecapacitor 120 are connected in the substrate 105 to block the currentleakage path into the substrate 105, which can effectively reducecurrent measurement noise as described above.

The process for building the antifuse OTP memory cells 900, 1000 shownFIGS. 9-10 can include similar steps as those described above inrelation to FIG. 8. The combination of lightly doped region (112) anddeeply doped regions (113, 113, 123, 134) is formed by applying thespacer structures 117, 127 between two doping processes.

Referring to FIG. 11, using the antifuse OTP memory cell 900/1000 as anexample, first, a gate dielectric layer is formed on a substrate (step1180). Gate dielectric and gate patterns are then produced for anantifuse capacitor and a select transistor on the substrate (step 1182).These can be accomplished using masking and etching techniques. Thingate dielectric layers are formed on the substrate for the antifusecapacitor and for the select transistor respectively. Then, the gatestacks are formed on top of the gate dielectric layers. The antifusecapacitor and the select transistor are defined by masking and etching.The combination of lightly doped regions and deeply doped regions isformed by applying the spacer structures 117, 127 between two dopingprocesses.

The relatively shallow doping region (112 in FIGS. 9 and 10) is formedon one side of the select transistor by masking antifuse capacitor 120and another side of the select transistor (step 184). The anti-punchthrough ion implant is eliminated for antifuse capacitor and anotherside of the select transistor with same masking process. Moreover,annealing steps maybe added after dopant implant process.

Next, spacers are built around the antifuse capacitor stack and theselect transistor gate stack (step 1186). Deeply doped regions (114 inFIGS. 9 and 10) on the other side of the select transistor 110, anddeeply doped regions (123, 124 in FIGS. 9 and 10) for the antifusecapacitor 120 are formed in masking process and ion implant (step 1188).Subsequent annealing can be conducted then drives deeply doped regionsto be underneath the gate dielectric. The extent of annealing can becontrolled such a degree to allow the deeply doped regions 123 and 124to electrically connect with each other (FIG. 10). The junctions arethus formed for the select transistor and the antifuse capacitor (1190).

The above description of the disclosed embodiments enables those skilledin the art to implement or use the present invention. Variousmodifications to these embodiments will be readily apparent to thoseskilled in the art, and the general principles defined herein may beapplied to other embodiments without departing from the spirit or scopeof the invention. Thus, the present invention will not be limited to theembodiments shown herein but will conform to the widest scope consistentwith the principles and novel features disclosed herein.

What is claimed is:
 1. An antifuse One-Time-Programmable memory cell,comprising: a substrate; a low-voltage select transistor formed on thesubstrate, comprising: a first gate dielectric layer formed on thesubstrate, wherein the first gate dielectric layer is thinner than 40nm, which enables a low-voltage select transistor device; a first gateformed on the gate dielectric layer; a first low-voltage junction formedin the substrate; and a second low-voltage junction formed in thesubstrate, wherein a source and a drain for the select transistor areformed by the first low-voltage junction and the second low-voltagejunction; and a hybrid antifuse capacitor formed on the substrate,comprising: a second gate dielectric layer formed on the substrate,wherein the second gate dielectric layer is thinner than 40 nm, whichenables a low-voltage antifuse capacitor device; a second gate formed onthe gate dielectric layer; a first high-voltage junction formed in thesubstrate; and a second high-voltage junction formed in the substrate,wherein a source and a drain for the antifuse capacitor are respectivelyformed by the first high-voltage junction and the second high-voltagejunction.
 2. The antifuse One-Time-Programmable memory cell of claim 1,wherein the first high-voltage junction and the second high-voltagejunction of the antifuse capacitor are separated by a channel in thesubstrate.
 3. The antifuse One-Time-Programmable memory cell of claim 1,wherein the first high-voltage junction and the second high-voltagejunction of the antifuse capacitor are electrically connected.
 4. Theantifuse One-Time-Programmable memory cell of claim 1, wherein the firstgate dielectric layer or the second gate dielectric layer is thinnerthan 30 nm.
 5. The antifuse One-Time-Programmable memory cell of claim1, wherein the hybrid select transistor is a low voltage deviceoperating at 3.3V or below.
 6. The antifuse One-Time-Programmable memorycell of claim 1, wherein the hybrid antifuse capacitor is a low voltagedevice operating at 3.3V or below.
 7. The antifuse One-Time-Programmablememory cell of claim 1, wherein the first high-voltage junction or thesecond high-voltage junction has a threshold junction breakdown voltageabove 3.3 Volt.
 8. The antifuse One-Time-Programmable memory cell ofclaim 7, wherein the second low-voltage junction and the firsthigh-voltage junction are electrically connected.
 9. The antifuseOne-Time-Programmable memory cell of claim 1, wherein the firstlow-voltage junction or the second low-voltage junction includes a firstdoped region adjacent to the first gate dielectric layer at a lowerdoped level than a second doped region away from the first gatedielectric layer.
 10. The antifuse One-Time-Programmable memory cell ofclaim 1, wherein the first high-voltage junction or the secondhigh-voltage junction includes a first doped region adjacent to thesecond gate dielectric layer at a lower doped level than a second dopedregion away from the second gate dielectric layer.